This invention relates to a protection circuit for smaller diameter gas discharge lamps and for compact fluorescent tube designs in particular. The device acts to reduce power in an electronic ballast and keeps the power at a low level when sensing a fault condition occurring in the lamp.
As fluorescent lamps age, the emissive material is worn away so that any arc current must flow from the bare tungsten filament. The work functions of the tungsten filament is high, therefore the filament is quickly melted. If the voltage supplied by the ballast is high enough, the lamp can be started with a broken or missing filament. After the filament has evaporated, the lamp arc current can flow from one of the filament support wires. These wires also have a high work function and considerable heat is generated when arc current flows from one of them. In some cases the heat is high enough to crack or melt the glass wall of the lamp. Overheating is a particular problem with small diameter lamp tubes and compact fluorescent lamps especially toward the end of the lamp's life. In a compact fluorescent lamp both ends of the lamp are adjacent to each other and are encased in a plastic housing. The base of the lamps are mounted in a plastic housing to which either a connector or a ballast is attached. When the lamps overheat, the hot glass can cause the plastic to deform. The glass can reach temperatures of over 350 degrees Celsius on the outer surface during abnormal lamp operating modes. It is possible that operation at these high temperatures will cause the glass wall to crack. It is desirable for the ballast, when operating, to be able to reliably control all lamp operating modes, so that abnormal lamp failure modes are not permitted to develop.
Three test schemes have been proposed as an IEC standard by Osram Sylvania; first, a test for maximum current heating in the electrodes, second, a test for lamp overvoltage or voltage rise and third, a test for lamp rectification. Designing a cost effective and manufacturable circuit to perform these tests is challenging. Various shutdown sensing and control circuit schemes are possible. One method is to sense when the lamp voltage rises above a setpoint while the lamp arc is ignited and then issue a shutdown or power reduction command to shut down the supply of power from the ballast to the lamp. A second possible solution is to sense when the output power rises above a control setpoint and then issue a command to reduce the supply of power from the ballast to the lamp. A third possible solution is to sense the filament current level when the filament voltage is present and issue a command if the filament current does not flow or has a zero value. Each of these solutions pose a challenge to the circuit designer. It is difficult to build a circuit that does not suffer from being complicated to build, and degrading the reliability of the ballast system by adding additional components that are subject to failure.
Once the ballast power is reduced to a safe level, the circuit needs to be able to perform two functions. First, it needs to be able to sense when a lamp is behaving abnormally and automatically resume normal operation when the failed lamp is replaced. The second function of a shutdown circuit could be described as limiting the periodic attempts at restarting until the lamp is replaced to minimize annoying blinking of the lamps until they are replaced. Another solution for this function, would be for the ballast to sense proper lamp operation and resume normal operations without input voltage reset or lamp blinking. The circuits could be designed to allow resetting only when power is first removed and then reapplied to the ballast. This would work in some applications, but not in others where the lamp fixture is normally continuously powered.
Prior attempts at gas discharge lamp ballasts with shutdown circuits have resulted in implementations with various drawbacks. Such implementations are illustrated by U.S. Pat. No. RE 32,901, RE 32,953, and 5,004,955. These show ballast shutdown circuits that sense an overvoltage condition in the resonant output circuit. They require a voltage clamp (varistor) to activate before the shutdown circuit can be activated. Its purpose is to prevent a shock hazard at the output terminals and not to protect the lamp. This approach has several draw backs, first two of the abnormal lamp failure modes may not be detected. The missing filament and asymmetrical operation. Second, these circuits require turning the power off and on after a new lamp is installed before the lamps will strike. For many large fixture installations on one circuit this is not feasible. U.S. Pat. 4,667,131, and 4,503,363 disclose shock protection circuits that have a two stage turn off. They use a high voltage SCR and a transistor to disable the inverter. These additional and high voltage parts add additional cost to the circuit. These circuits will not detect asymmetrical operation of the lamp. U.S. Pat. 5,436,529 shows a circuit that disables an inverter in response to an overvoltage condition occurring at the lamp. Once turned off, it stays off until the defective lamp is replaced and then automatically restarts. It requires extra components such as DC blocking capacitors to allow the circuit to sense a broken filament and turn-off.
These prior art circuits require a large component count and their associated complexity. The prior art shutdown and lamp protection circuits are all directed toward series-resonant non-isolated ballast designs that do not have an isolation transformer between the lamp and the circuit. It is advantageous to be able to have a lamp protection circuit that operates with a parallel resonant isolated ballast. This is the preferred ballast and lamp configuration in the industry.
A currently unmet need exists for a simple circuit to work with electronic ballasts that are of a parallel resonant design with an isolation transformer to protect small diameter gas discharge lamps and compact fluorescent lamps from overheating and cracking.